Bias tires having relatively narrow cross sectional widths were predominantly used in vehicles up to around 1960 because vehicles in those days were relatively lightweight, had relatively low cruising speed required thereof and thus did not put so much stress on the tires. However, radial tires having wide and flat structures are predominant these days because good driving stability in high speed running, as well as good wear resistance, is required of tires as highway networks are developed and vehicle speed increases (e.g. PTL 1).
However, increasing widths of tires decreases free space in a vehicle and deteriorates comfortablility therein. This is becoming a big issue because electric vehicles which have been developed for use in recent years, in particular, must have sufficient space for accommodating driving units such as a motor for controlling torque of rotating tires around drive shafts and in this regard ensuring sufficient space in vicinities of tires thereof is increasingly important.
Further, there has been increasingly a demand for a better fuel efficiency in recent years as people are more concerned about environmental issues. It has been conventionally known that increasing diameter and width of a tire is effective in terms of decreasing rolling resistance value (RR value) of the tire for better fuel efficiency thereof. Increasing diameter and width of a tire, however, also increases weight of the tire and air resistance of a vehicle, thereby resulting in an increase in resistance experienced by the vehicle and too much load on the tire.
Moreover, increasing diameter of a tire increases belt tension, enhances ring rigidity of the tire and thus makes the tire sensitive to an input of force from a road surface, thereby causing another problem of deteriorated low-noise properties of the tire.